Tidying up, applying opacity if it exists in styles

examples/dev_sandbox.py

@@ -18,6 +18,7 @@ class BasicApp(App):
         self.bind("tab", "toggle_class('#sidebar', '-active')")
         self.bind("a", "toggle_class('#header', '-visible')")
         self.bind("c", "toggle_class('#content', '-content-visible')")
+        self.bind("d", "toggle_class('#footer', 'dim')")
 
     def on_mount(self):
         """Build layout here."""

examples/dev_sandbox.scss

@@ -34,7 +34,6 @@ Widget:hover {
 
 #header {
   text: $text on $primary;
-  opacity: 0.2;
   height: 3;
   border-bottom: hkey $secondary;
 }
@@ -53,7 +52,12 @@ Widget:hover {
 }
 
 #footer {
+  opacity: 1;
   text: $text on $primary;
   height: 3;
   border-top: hkey $secondary;
 }
+
+#footer.dim {
+  opacity: 0.5;
+}